Light-emitting diodes (LEDs) are good candidates to replace incandescent and other light sources. LEDs have higher power-to-light conversion efficiencies than incandescent lamps and longer lifetimes. In addition, LEDs operate at relatively low voltages, and hence, are better adapted for use in many battery-powered devices. Furthermore, LEDs are point sources, and hence, are better adapted than fluorescent sources for lighting systems in which a point light source that is collimated or focused by an optical system is required.
To compete with incandescent lights, the output spectrum of the LED must be altered to provide a spectrum that is perceived as being “white” by a human observer. In general, LEDs generate light in a small band of wavelengths. Hence, to build a light source that is perceived as being white, either the light from a number of LEDs of differing output wavelengths must be combined or light from a monochromatic LED must be down converted by a phosphor layer to provide light in additional regions of the visual spectrum. The most common form of white LED utilizes a blue-emitting LED and a layer of phosphor that converts part of the blue light into yellow light. The combination of blue and yellow light is perceived by a human observer to be white if the ratio of blue to yellow light is properly chosen.
Heat dissipation is a significant problem in LED-based lighting systems that are to compete with incandescent and fluorescent light sources. Unlike incandescent lights, LEDs must be run at relatively low temperatures. First, the efficiency with which an LED coverts electrical power to light decreases as the temperature of the LED increases. Second, the phosphor-conversion layers are typically constructed by dispersing phosphor particles in an epoxy layer that overlies the LED. When the blue light is converted to yellow light by the phosphor particles, the difference in energy between the blue photons and the yellow photons becomes heat that is deposited in the phosphor particle. This heat must pass through the phosphor layer before being dissipated to the ambient environment. Since the base material in which the phosphor is suspended has poor thermal conductivity, the temperature of the phosphor particles must be significantly above ambient to drive the heat through this layer. This heating often leads to structural failure in the phosphor layer due to the difference in thermal expansion coefficient between the carrier material and the phosphor particles. It should be noted that the fraction of the blue light that is converted to heat is significant. Hence, as the power output of the light sources increases, the problems associated with phosphor layer degradation also increases.